1. Field of the Invention
The present invention relates to an apparatus and method for reducing noise in an image, and a computer-readable storage medium storing software for reducing noise in an image.
2. Description of the Related Art
In photographing apparatuses such as a digital still camera and a video camera, noise is typically present in image data that is generated based on an electrical signal output from an imaging device which converts optical data into an electrical signal. Such noise can thus deteriorate the image. Thus, in some cases, the image data is filtered in order to reduce noise.
For example, Japanese Patent Publication No. 2006-311501 (the '501 publication) and Japanese Patent Publication No. 2005-142891 (the '891 publication) disclose technologies for reducing noise of image data. In the device described in the '501 publication, an average pixel value of a plurality of neighboring pixels that are symmetrically disposed about a target pixel of image data is restricted to a predetermined range, and low-pass-filtering is performed on the target pixel when the restricted pixel value of the neighboring pixels is set as a new pixel value. In the device described in the '891 publication, at least one value representing a degree of flatness or a degree of gradation is calculated in each pixel. The calculated value functions as a coefficient for performing planarization filtering on a pixel if the calculated value represents a flat portion, functions as a coefficient for performing edge enhancement filtering on a pixel if the calculated value represents a gradation portion, and functions as a coefficient for variably performing planarization filtering and edge enhancement filtering on a pixel if the calculated value represents an intermediate portion.
In the device described in the '501 publication, noise reduction may be achieved regardless of whether a target pixel corresponds to an edge portion, a flat portion, or a gradation portion. However, signals in the edge portion are excessively reduced due to filtering. Also, since the flat portion and gradation portion are not identifiable from each other, a noise reduction strength optimized for the flat portion is insufficient for the gradation portion. On the other hand, a noise reduction strength optimized for the gradation portion is excessive for the flat portion.
In the device described in the '891 publication, a gradation portion is often misrecognized as an edge portion. In this case, noise of the gradation portion is increased. Also, when the gradation portion is misrecognized as a flat portion, a noise reduction strength optimized for the flat portion is insufficient for the gradation portion. On the other hand, a noise reduction strength optimized for the gradation portion is excessive for the flat portion.